Fixes linking issue on OS X 10.7

This fixes the following linking issue on OS X 10.7.

```
Undefined symbols for architecture x86_64:
  "_CFRelease", referenced from:
      _ca_get_str in ao_coreaudio_properties.c.10.o
  "_CFStringGetCString", referenced from:
      _ca_get_str in ao_coreaudio_properties.c.10.o
  "_CFStringGetLength", referenced from:
      _ca_get_str in ao_coreaudio_properties.c.10.o
  "_CFStringGetMaximumSizeForEncoding", referenced from:
      _ca_get_str in ao_coreaudio_properties.c.10.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```